Scrape and wash the mussels well. Put them in a pot with a knob of butter, the finely chopped onions and wine. Boil on high heat until they open, then remove them. Take out the mussels and save the water.
Remove 1 shell half from each mussel and put the other shells with the meat in a deep dish. Strain the water from the boiling through a dense filter and put it on the heat once more. Dilute the flour with 1 tablespoon melted butter and gradually add it to the water.
Simmer for a few minutes, sprinkle it with black pepper and salt to taste. Pour this sauce over the mussels. Before serving, sprinkle the mussels with finely chopped parsley on top.